1. Field of the Invention
The invention relates generally to the field of subsurface safety valves used in wellbores. More specifically, the invention relates to magnetically sprung, magnetically geared valve actuation devices for use with such safety valves.
2. Background Art
Wellbore valves, such as subsurface safety valves, are used for controlling flow within a well tubing string suspended within a wellbore. Typically, valves of this type include a valve member that is pivotally mounted within the bore of a tubular body disposed within the well tubing string, for movement between open and closed positions. The valve member is urged by a spring to its closed position, but is arranged to be moved to the open position in response to the supply of hydraulic fluid pressure from a remote source at the Earth's surface. The hydraulic fluid pressure acts on a piston forming part of or associated with the valve member. Ordinarily, the valve member is arranged to close automatically under the force of the spring in response to the exhaust of such hydraulic fluid pressure, for example, in the event of failure of a monitored condition in or about the well.
Many such valves are tubing safety valves wherein the body of the valve is disposed within the well tubing string for controlling flow therethrough. Such a valve may be of a type in which the valve body is retrievable from within the tubing string, or in which the body is connected as part of the tubing string—i.e., tubing mounted. Typically, the valve is a flapper pivotally mounted in the bore of the valve body and arranged to be moved to the open position by a flow tube with which the piston is cooperates to move the flow tube within the bore. Thus, the supply of hydraulic fluid pressure lowers the flow tube to force the flapper to the open position. The spring acts on the piston to raise the flow tube and thus permit the flapper to close upon the exhaust of hydraulic fluid pressure on the piston.
U.S. Pat. No. 3,799,258 shows a typical tubing mounted valve of this type wherein the piston is an annular piston disposed about the flow tube within an annular pressure chamber between the flow tube and valve body. The piston urged to its closed position, enabling the flapper to close, by means of a coil spring compressed between the valve body and the flow tube. However, when valves of this type are installed at great depths, it is difficult for a coil or similar metal spring of acceptable size and strength to overcome the hydrostatic head of the hydraulic fluid in the control line leading to the pressure chamber, and thus raise the flow tube to permit the flapper to close.
U.S. Pat. No. 4,161,219 describes one proposed way to solve this problem by the use of piston which, as compared with the annular piston shown in U.S. Pat. No. 3,799,358, is disposed within a cylinder formed in the valve body to one side of the flow tube and is engageable at its lower end with the flow tube to move it to its lower position in response to supply of the hydraulic fluid to the cylinder. Thus, it was thought possible to reduce the cross-sectional area of the piston, and thus the downward force on the flow tube due to the hydrostatic pressure of the control fluid. The force reduction was believed sufficient to enable the flow tube to be raised, and thus permit the flapper to close upon exhaust of the control fluid, using a smaller coil spring than was previously required.
As there is a significant area change between concentric or annular pistons and side pistons at intermediate depths, the designer is faced with the decision of using multiple pistons or weaker springs. Multiple pistons are expensive and weaker springs are less safe. Moreover, metal springs are subject to fatigue and possible failure. The spring(s) must be able to survive and operate correctly over a period of as long as 20 years. There continues to be a need for wellbore valves that overcome some of the limitations of valve structures known in the art.